Typical precision amplifiers for industrial driver applications are linear amplifiers with a class AB output stage. They provide accurate signal processing at low output noise, at the expense of poor power efficiency. The dissipated power usually even exceeds the load power. Examples are the XTR300 and XTR111 precision amplifiers of Texas Instruments Incorporated.
Class D amplifiers and switching DC-DC converters have a high efficiency of typically more than 80%, but have a low bandwidth (a fraction of the switching frequency) and require a large inductor and capacitor at the output. They also have high output noise related to the switching frequency. An example is the TPA3001 class D amplifier of Texas Instruments Incorporated.
To achieve high accuracy and high speed at high efficiency, a switch-mode assisted linear amplifier has been proposed by Ertl et al., “Basic considerations and topologies of switch-mode assisted linear amplifiers”, IEEE Transactions on Power Electronics, Vol. 44, No. 1, February 1997. This concept uses a linear amplifier in parallel with a class D amplifier. The class D amplifier is controlled by the output current of the linear amplifier, which allows unloading a low-frequency fraction of the linear amplifier output current. However, this concept needs a large inductor and load capacitor for operation. A large inductor limits the slew rate and efficiency with a highly dynamic load as the dI/dt through the inductor is limited, just as the dV/dt across a large capacitor. In addition, if used as an analog signal driver for industrial applications, the load is complex, i.e. resistive and capacitive, and long wires even may add inductance. The required output may be constant voltage or current. A class D amplifier, even in the context of a switch-mode assisted linear amplifier, would have great difficulty to handle the large variety of complex loads.